Epitaxial Lateral Overgrowth Research Articles

AlxGa1−xN GROWTH BY Si/N TREATMENT OF SAPPHIRE SUBSTRATE: COMPARISON WITH OTHERS GROWTH TECHNIQUES

AlxGa[Formula: see text]N films were grown on Si/N-treated sapphire substrate by atmospheric pressure metalorganic vapor phase epitaxy (AP-MOVPE) in a home-made vertical reactor. This process can be considered as randomly in situ epitaxial lateral overgrowth (ELO) technology. The growth firstly begins by three-dimensional (3D) mode and is completed in two-dimensional (2D) growth mode as shown by real time in situ laser [Formula: see text][Formula: see text]nm[Formula: see text] reflectometry measurements and confirmed by scanning electron microscopy (SEM) images. Secondary ion mass spectroscopy (SIMS) measurements evidence Al composition pulling effect in the AlxGa[Formula: see text]N layer. The Si/N treatment technique is compared to conventional AlxGa[Formula: see text]N growth techniques. The results of high-resolution X-ray diffraction (HRXRD), photoluminescence (PL) measurements and SEM images agree well on the fact that the Si/N treatment produces AlxGa[Formula: see text]N layers with comparable qualities of AlxGa[Formula: see text]N layers grown on high temperature GaN template but with much higher qualities than AlxGa[Formula: see text]N layers grown on low temperature AlN nucleation layer. Moreover, the Si/N treatment technique permits the growth of high quality AlxGa[Formula: see text]N layers with appreciable thicknesses with respect to the others techniques.

The morphologies of GaN crystals grown on Ga- and N-face of HVPE seeds by the Na flux liquid phase epitaxial method

GaN crystal growth on Ga-face and N-face of HVPE-GaN seeds was investigated using Na flux liquid phase epitaxial (LPE) method, respectively. The phase structure and morphology of as-grown GaN crystals were characterized using X-ray diffraction (XRD) and the scanning electron microscope (SEM), respectively. Under the growth temperature of 850 °C and keeping nitrogen pressure 3.0 MPa for 150 h, the typical pyramidal GaN poly-crystals with wurtize structure grew by spontaneous nucleation growth on the Ga-face of GaN -HVPE seeds. In contrast, GaN single crystals could be obtained by epitaxial growth on the N-face of GaN-HVPE seeds. From the observation of the side view of as-grown GaN crystals grown on the N-face, there were two apparent stages along the growth direction (-c direction) during the crystal epitaxial growth process. The typical hexagonal layered structure of as-grown GaN crystals occurred in the initial stage, and then the typical hexagonal pyramidal structure of it occurred in the second growth stage. In addition, from the observation of optical microscope, the as-grown GaN crystal with the initial growth stage only presented different morphology from that with the two-growth stage. The continuous lateral equilibrium epitaxial growth on N-face of GaN crystal seed may be of great significance for the growth of large size GaN single crystal.

Stress study of GaN grown on serpentine-channels masked Si(111) substrate by MOCVD

Abstract An advanced epitaxial lateral overgrowth (ELOG) structure named “serpentine-channels masked Si(111) substrate” has been introduced, which only has one high-defect region per period compared with the conventional ELOG method. We successfully obtained both coalesced and uncoalesced GaN layers on the substrate simultaneously by the metal-organic chemical vapor deposition (MOCVD) method. The stress states of these two kinds of GaN epilayers were investigated by room-temperature micro-Raman scattering technique. The stress level in the uncoalesced GaN layer was a little lower than that in the coalesced GaN layer. Raman spectra reveal the periodical variations of residual tensile stress in GaN by analyzing E2 (high) phonon mode. In addition, thermal stress distribution of GaN was simulated by elasticity theory using the finite-element method (FEM). The results of simulation are entirely consistent with the experimental results derived from micro-Raman measurements.

Non-edge-triggered inversion from Ga polarity to N polarity of c-GaN domains on an SiO2 mask during epitaxial lateral overgrowth

It was previously reported that N-polar c-GaN domains nucleated in window openings on c-plane sapphire were inverted to Ga-polar domains at the edge of an SiO2 mask during epitaxial lateral overgrowth, but it was asserted that polarity inversion of N-polar GaN domains could not occur beyond the edge of the SiO2 mask. However, that assertion was demonstrated only in the case of a-facet-exposed GaN. It is reported here that polarity inversion from Ga polarity to N polarity of m-facet-exposed c-GaN domains occurred during epitaxial lateral overgrowth on the flat region beyond the edge of a circular-patterned SiO2 mask. An increased flow rate of NH3 during the epitaxial lateral overgrowth is thought to induce this type of non-edge-triggered polarity inversion. Further investigation reveals that non-edge-triggered polarity inversion is also possible when the a facet is exposed at the lateral growth front of Ga-polar GaN domains.

Distributed Bragg Reflectors for GaN-Based Vertical-Cavity Surface-Emitting Lasers

A distributed Bragg reflector (DBR) is a key building block in the formation of semiconductor microcavities and vertical cavity surface emitting lasers (VCSELs). The success in epitaxial GaAs DBR mirrors paved the way for the ubiquitous deployment of III-V VCSELs in communication and mobile applications. However, a similar development of GaN-based blue VCSELs has been hindered by challenges in preparing DBRs that are mass producible. In this article, we provide a review of the history and current status of forming DBRs for GaN VCSELs. In general, the preparation of DBRs requires an optimization of epitaxy/fabrication processes, together with trading off parameters in optical, electrical, and thermal properties. The effort of epitaxial DBRs commenced in the 1990s and has evolved from using AlGaN, AlN, to using lattice-matched AlInN with GaN for DBRs. In parallel, dielectric DBRs have been studied since 2000 and have gone through a few design variations including epitaxial lateral overgrowth (ELO) and vertical external cavity surface emitting lasers (VECSEL). A recent trend is the use of selective etching to incorporate airgap or nanoporous GaN as low-index media in an epitaxial GaN DBR structure. The nanoporous GaN DBR represents an offshoot from the traditional epitaxial approach and may provide the needed flexibility in forming manufacturable GaN VCSELs. The trade-offs and limitations of each approach are also presented.

Study of AlN based materials grown on nano-patterned sapphire substrates for deep ultraviolet LED applications

High quality and crack-free AlN films were obtained by using nano-patterned sapphire substrates (NPSS) grown at AMEC Prismo HiT3TM MOCVD platform. It is believed that the introduced epitaxial lateral overgrowth can annihilate most of dislocations and the grain boundary induced tensile stress can be significantly suppressed by NPSS. For a 5 μm thick AlN film, FWHMs of 173 arcsec and 335 arcsec were observed from AlN (002) and (102) X-ray rocking curves, respectively, indicating the high crystalline quality. The surface of AlN films grown on NPSS shows a typical step-bunching morphology with atomic steps on the bunched terrace. In addition, we also obtained excellent thickness uniformity for AlN films grown on NPSS with within-wafer and wafer-to-wafer thickness uniformity of 0.69% and 0.92%, respectively.

Effects of Au catalyst geometry on Ge films grown laterally on Si using the vapor–liquid–solid mechanism

A lateral epitaxial growth technique has been demonstrated to grow high-quality Ge micro-films on Si at low temperatures using the vapor–liquid–solid mechanism. These Ge films were grown within a confined structure to foster horizontal growth in the presence of Au catalyst. In this work the size and geometric shape of the Au catalyst were shown to impact the film size and morphology. In particular, a general trend was observed, as the size of Au catalyst increased the Ge film size initially increased as well until the Ge film size plateaued and eventually decreased. This phenomenon is described by a model, which is a function of Au catalyst size and GeH4 dose and can be used to estimate the final Ge film size. In general, a large-sized Au catalyst absorbs more Ge because of its large vapor–liquid interface but it also requires more Ge to saturate the liquid, and therefore this fact results in a peak in the plot of the Ge film size versus the Au size. In addition, a large GeH4 dose due to more absorption of GeH4 leads to multiple nucleation sites in a small Au catalyst, while a micro-crucible with a larger Au volume or reduced vapor–liquid interface can help to realize single nucleation. These results helped in designing two-opening Au catalysts with asymmetric sides, which preferentially nucleates a large crystal near the wider side.

High-Quality GaAs Planar Coalescence over Embedded Dielectric Microstructures Using an All-MBE Approach

We demonstrate for the first time an entirely molecular beam epitaxy (MBE) approach to high-quality GaAs planar coalescence over embedded dielectric microstructures. Specifically, an all-MBE approach was achieved by developing a new two-stage growth process, merging the MBE growth regimes of III-flux modulated lateral epitaxial overgrowth (LEO) with self-ordered planarization of nonplanar substrates to produce highly selective planar coalescence specifically for embedding [010]-aligned silica gratings patterned on (001) substrates. The resulting planar coalescence returned a smooth (001) surface with surface roughness as low as 3 nm root-mean-square and photoluminescence (PL) equivalent to grating-free controls. In demonstrating high-quality GaAs coalescence, we also report for the first time an intentionally enhanced single InGaAs/GaAs/AlAs quantum well PL test structure seamlessly grown directly above embedded silica gratings, leading to a 1.4× enhancement in PL as a result of both Purcell and extractio...

GaN microstructure light-emitting diodes directly fabricated on tungsten-metal electrodes using a micro-patterned graphene interlayer

We report on the selective-area growth of GaN microstructures on tungsten (W)-metal electrodes using a micro-patterned graphene intermediate layer between GaN and W, and demonstrate their use as light-emitting diodes (LEDs). Prior to the GaN growths, the cm-scale graphene layer was transferred on W and then further patterned into a regular array of few-μm-sized graphene microdots using conventional lithography. The graphene microdots served as a seed layer for selectively growing crack-free GaN microstructures with regular diameter and spacing. Each microstructure displayed a microdisk morphology, exhibiting a single crystalline phase from epitaxial lateral overgrowth (ELOG). We observed ohmic behavior between the as-grown GaN microdisks and underlying W film, facilitating the fabrication of LED microarrays. Using the underlying W layer as an ohmic contact, we fabricated p-n junction GaN microdisk LEDs, consisting of three periods of InxGa1–xN/GaN multiple quantum wells. Uniform electroluminescence was observed across the microdisks. These results open up novel strategies for streamlined fabrication of high-performance and high-resolution LEDs.

Effects of Growth Temperature on Morphology of GaN Crystals by Na Flux Liquid Phase Epitaxial Method

The morphology of GaN crystals grown in a temperature range of 800–860°C by the Na flux liquid phase epitaxial (LPE) method was observed by the optical microscope and the scanning electron microscope (SEM). Significant changes in the morphology of as-grown GaN crystals occurred with the growth temperature being increased. Differing from the crystals grown at the temperature lower than 840°C, the side morphology of crystals grown at 850°C and 860°C presented a rather integrated and flat facet observed along the m-direction. In addition, there were two distinct growth stages with lateral equilibrium epitaxial growth (Stage I) and c direction growth (Stage II) during the epitaxial growth process. Under the growth temperature of 850°C and keeping nitrogen pressure 3 MPa for 150 h, the as-grown GaN crystal had relatively higher crystalline quality, the thickness and yield of GaN-LPE crystal were about 622 μm and 15.2%, respectively. The dislocation density in the GaN crystal was also remarkably reduced to the order of 105–106 cm−2. Therefore, the continuous lateral equilibrium epitaxial growth (stage I) is feasible to achieve a large size and high quality GaN single crystal.

Optical and interface properties of direct InP/Si heterojunction formed by corrugated epitaxial lateral overgrowth

We fabricate and study direct InP/Si heterojunction by corrugated epitaxial lateral overgrowth (CELOG). The crystalline quality and depth-dependent charge carrier dynamics of InP/Si heterojunction are assessed by characterizing the cross-section of grown layer by low-temperature cathodoluminescence, time-resolved photoluminescence and transmission electron microscopy. Compared to the defective seed InP layer on Si, higher intensity band edge emission in cathodoluminescence spectra and enhanced carrier lifetime of InP are observed above the CELOG InP/Si interface despite large lattice mismatch, which are attributed to the reduced threading dislocation density realized by the CELOG method.

GaN-based Vertical-Cavity Surface-Emitting Lasers Incorporating Dielectric Distributed Bragg Reflectors

This paper reviews past research and the current state-of-the-art concerning gallium nitride-based vertical-cavity surface-emitting lasers (GaN-VCSELs) incorporating distributed Bragg reflectors (DBRs). This paper reviews structures developed during the early stages of research into these devices, covering both major categories of GaN-based VCSELs: hybrid-DBR and all-dielectric-DBR. Although both types exhibited satisfactory performance during continuous-wave (CW) operation in conjunction with current injection as early as 2008, GaN-VCSELs have not yet been mass produced for several reasons. These include the difficulty in controlling the thicknesses of nitride semiconductor layers in hybrid-DBR type devices and issues related to the cavity dimensions in all-dielectric-DBR units. Two novel all-dielectric GaN-based VCSEL concepts based on different structures are examined herein. In one, the device incorporates dielectric DBRs at both ends of the cavity, with one DBR embedded in n-type GaN grown using the epitaxial lateral overgrowth technique. The other concept incorporates a curved mirror fabricated on (000-1) GaN. Both designs are intended to mitigate challenges regarding industrial-scale processing that are related to the difficulty in controlling the cavity length, which have thus far prevented practical applications of all-dielectric GaN-based VCSELs.

Characterization of high-quality relaxed flat InGaN template fabricated by combination of epitaxial lateral overgrowth and chemical mechanical polishing

We report the characterization of thick completely relaxed InGaN layers grown using epitaxial lateral overgrowth (ELO) with a facet structure. ELO-InGaN layers were successfully grown on a stripe-patterned GaN template. The full width at half maximum of the X-ray rocking curve of ELO-InGaN on the InGaN templates was smaller than that of non-ELO InGaN. Because of the difficulty in improving the morphology of ELO-InGaN layers grown on patterned GaN templates, chemical and mechanical polishing (CMP) was performed to obtain a flat c-plane surface. The photoluminescence intensity of InGaN/GaN multiple quantum wells (MQWs) on the CMP-ELO-InGaN template was approximately two times stronger than that of MQWs on the GaN template. Furthermore, the peak wavelength of the MQWs on the CMP-ELO-InGaN template was longer than that on the GaN template because of the high In pulling effect.

High crystalline quality heteroepitaxial diamond using grid-patterned nucleation and growth on Ir

High crystalline quality heteroepitaxial diamond films were successfully formed by using grid-patterned growth on Ir with dc plasma CVD. The crystalline qualities were evaluated by X-ray diffraction, Raman spectroscopy, and etch pit method. Flat {100} diamond films without unepitaxial crystals were formed through epitaxial lateral overgrowth with grid-patterned nucleation region of 100-μm spacing. Omega-rocking curves using X-ray diffraction showed obvious improvement in tilt and twist components for wider grid-spacing. For 100-μm grid spacing, the lowest tilt and twist were 0.064 and 0.043°, respectively. The average dislocation density of 9 × 106 cm−2 for the diamond film grown with grid spacing of 100 μm was the lowest ever in reported values for heteroepitaxial diamond. A correlation between the distribution of crystallinity and dislocation was discussed from the results of etch pit method, Raman mapping and TEM observation. Finally, the crystallinity was compared with single crystal diamonds grown by CVD and HPHT. By using grid-patterned nucleation, the quality of heteroepitaxial diamond on Ir has reached in the range of homoepitaxial diamond film.

Enhancing diamond NV center density in HPHT substrate and epitaxy lateral overgrowth layer by tungsten pattern

The nitrogen vacancy (NV) center distribution in epitaxial lateral overgrowth (ELO) single crystal diamond layer grown on tungsten patterned HPHT substrate by microwave plasma chemical vapor deposition (CVD) system has been investigated. It has been found that in ELO diamond layer densities of NV0 and NV− center above the tungsten metal are enhanced. Meanwhile, in patterned high-pressure and high-temperature (HPHT) substrate the density of NV− center beneath the tungsten metal is much higher than that of NV0. The HPHT substrate does not contain NV centers before CVD growth, and there is almost no NV center in the region without tungsten metal after growth.

Epitaxial lateral overgrowth of α-Ga2O3 by halide vapor phase epitaxy

We demonstrate the epitaxial lateral overgrowth of α-Ga2O3 by halide vapor phase epitaxy. We prepared patterned SiO2 masks on a (0001) α-Ga2O3/sapphire template, and then α-Ga2O3 islands were regrown selectively on the mask windows. The islands grew vertically and laterally to coalesce with each other. Facet control of the α-Ga2O3 islands was achieved by controlling the growth temperature, and inclined facets developed by decreasing the temperature. Transmission electron microscopy revealed that the crystal quality of the regrown α-Ga2O3 was improved owing to both the blocking of dislocations by the mask and the dislocation bending by the inclined facets.

New Relaxation Mechanism Enabling High-Quality, Laterally Grown Ge on Si

We demonstrate a novel growth technique, metal-catalyzed, lateral epitaxial growth, to grow Ge films laterally on Si with reduced threading dislocation densities (TDD). In contrast to traditional planar film growth on Si, this technique starts with a small crystal nucleation at a specific position on Si followed by Ge lateral film growth. In plan-view and cross-sectional transmission electron microscopy micrographs, high-density threading dislocations were found to be present in the initial growth areas, while the lateral overgrowth areas demonstrated substantially reduced TDD or even defect-free areas. Furthermore, the X-ray diffraction results showed that the films were fully relaxed and mostly pure Ge. Full relaxation and low TDD Ge films are two surprising results given that the growth occurred at a low temperature (375 °C), which cannot be understood with the current misfit dislocation nucleation and glide model of film relaxation. For these reasons, we suggest the presence of a new relaxation mechan...

Epitaxial lateral overgrowth of GaN on nano-cavity patterned sapphire substrates

The epitaxial lateral overgrowth of GaN by metal-organic chemical vapor deposition using a nano-cavity patterned sapphire substrate (NCPSS) was investigated. The NCPSS, with a hexagonal non-close-packed nano-cavity pattern on the sapphire substrate, was fabricated by polystyrene sphere coating and size reduction by reactive ion etching, followed by deposition of alumina and thermal oxidation. The coalescence of GaN on the NCPSS was achieved by the formation of relatively large GaN islands and enhanced lateral overgrowth of the GaN islands over several nano-cavity pattern areas. The threading dislocation density (TDD) measured by cathodoluminescence measurement was significantly reduced from 2.4 × 108 cm−2 to 6.9 × 107 cm−2 by using the NCPSS. A dislocation behavior that contributes to the reduction of TDD of the GaN layer was observed by transmission electron microscopy. Raman spectroscopy revealed that the compressive stress in the GaN layer was reduced by 21% due to the embedded nano-cavities. In addition, the diffuse reflectance of GaN on the NCPSS was enhanced by 54% ∼ 62%, which is attributed to the increased probability of light extraction through effective light scattering by nano-cavities.

Reducing stimulated emission threshold power density of AlGaN/AlN multiple quantum wells by nano-trench-patterned AlN template

We investigate the crystal quality of epitaxy lateral overgrowth (ELOG) AlN layers and the performance of optical pumping AlGaN-based deep-ultraviolet laser diodes (LDs) on two different patterned AlN templates upon sapphire substrate. The full width at half maximum values of (0002) and (101¯2) X-ray diffraction rocking curves of the ELOG AlN layer on the nano-net-patterned AlN template are 76 and 306 arcsec, and the values of that on the nano-trench-patterned AlN template are 114 and 357 arcsec, respectively. Nevertheless, the threshold power density (Pth) of the 272-nm lasing from the multiple quantum wells (MQWs) on the nano-trench-patterned AlN template is 11% lower than that on the nano-net-patterned AlN template. The reason is that the continuous low threading dislocation density (TDD) MQWs zones above the ELOG coalescent areas upon trenched patterns are in parallel with the stimulated light emission direction. When light resonates through the continuous low TDD MQWs zones, it has higher optical gain and less non-radiative recombination than the case on the nano-net-patterned AlN template where the low TDD MQWs zones are discontinuous on the light path. The results indicate that not only the crystal quality but also the TDD distribution originated from the ELOG on the patterned templates have crucial influence on the Pth, and trench-patterned template can improve the performance of waveguide LDs with Fabry-Perot cavities.

High-quality semipolar GaN grown on carbon nanotube-patterned sapphire by hydride vapor phase epitaxy

High-quality semipolar GaN is successfully grown on carbon nanotube-patterned sapphire (CNTPS) by hydride vapor phase epitaxy (HVPE). The root-mean-square (RMS) roughness of an as-grown sample is less than 0.8 nm with a scan area of 5 × 5 µm2. The full widths at half maximum (FWHMs) of on-axis X-ray rocking curves are 453 arcsec rocking toward the direction and 485 arcsec rocking toward the direction. Semipolar GaN grown on CNTPS has shown better isotropy than that grown on m-plane sapphire so far. The defect density is significantly reduced owning to epitaxial lateral overgrowth (ELOG). It is worth noting that this technique is not only nonlithographic but also cost-effective for mass production compared with conventional ELOG.